In a typical acoustic cloaking condition the plane waves from a distant enemy sonar source arrive at the object and emerge from the opposite side, reflect or scatter from particles of the medium and any other reflecting objects such as the ocean surface or bottom, and return back to the sonar source as if there were no targeted object in the path of the waves. With passive cloaking the waves appear to bend around the object as if the object were not there. One form of passive cloaking involves the provision of special materials for the object.
There presently exist efforts to develop inactive metamaterials for cloaking spherical (or other shape) shells so as to both eliminate backscattering and fill in the shadow zone behind the shell. An analytical model for this has been given by Cummer et al. [“Scattering Theory Derivation of a 3D Acoustic Cloaking Shell,” Phys. Rev. Lett. 100, 024301 (2008)], which develops the case for an ideal spherical shell. The development of such cloaking materials for spherical and other shapes is meant to inhibit the acoustic detection of objects, such as mines, torpedoes, UUV's and, ultimately, submarines by making the object invisible to acoustic waves.
This form of inactive cloaking, however, covers the target in a way that shields the target from using its own acoustic sonar means for detecting the source, unless the cloaking is turned off. Although it may be possible to eventually develop active metamaterials, which could be turned on or off, the present invention proposes an alternative approach.
Accordingly, it is an object of the present invention to provide an active cloaking transducer system which effectively cloaks the target and yet can also be used as a conventional active, acoustic sonar source and receiver.